1. Field of the Invention
The invention relates to a fuel cell, and more particularly concerns a fuel cell able to suppress a decline in performance caused by a fuel deficiency.
2. Description of the Related Art
A fuel cell generates electrical energy through an electrochemical reaction in a membrane electrode assembly (hereinafter “MEA”), which includes an electrolyte membrane and electrodes (i.e., an anode and a cathode) arranged on both sides of the electrolyte membrane. The electrical energy that is generated is then extracted from the MEA via separators arranged on both sides of the MEA. Among the various types of fuel cells that exist today, polymer electrolyte fuel cells (hereinafter “PEFC”), which are used in home cogeneration systems and automobiles and the like, are able to operate in a low temperature range. PEFCs are also receiving much attention as power supplies that are ideal for electric vehicles and as mobile power supplies because they exhibit high energy-conversion efficiency, have a short startup time, and the systems are small and lightweight.
A single cell of a PEFC includes an electrolyte membrane and an anode and a cathode. Both the anode and the cathode each have at least a catalyst layer. The theoretical electromotive force of a single cell is 1.23 volts. However, because this low electromotive force is insufficient for power supplies for electric vehicles and the like, single cells are normally stacked together in series to form a stack. End plates or the like are then arranged on both ends of the stack in the stacking direction to form a stacked PEFC. A tightening pressure is applied from both ends, in the form of a stacked PEFC, to reduce contact resistance.
The electrochemical reaction that generates electricity in the PEFC progresses in the following stages for example. First, hydrogen delivered to the anode is broken down into hydrogen ions and electrons in the presence of a catalyst (such as platinum supporting carbon; hereinafter platinum may also be referred to as “normal catalyst”).Anode reaction: H2→2H++2e−The hydrogen ions (hereinafter also referred to as “proton”) that are freed then travel to the cathode by passing through an electrolyte membrane, which conducts ions when moist. Because the electrolyte membrane only allows ions to pass through, the freed electrons, which are unable to pass through the electrolyte membrane, travel to the cathode via an external circuit. It is the movement of electrons by which the fuel cell generates electricity. Meanwhile, water is produced by the reaction of oxygen, which is delivered to the cathode, with the electrons and protons that have traveled to the cathode.Cathode reaction: 2H++2e−+(½)O2→H2O
When a PEFC operates, the interior of the cells are in various gas states so the anode may be exposed to a high potential state (such as a potential state of 1.6 V). In a high potential state, the constituent material of the anode (such as Pt, C, etc.) degrades, which reduces the performance of the PEFC. Therefore, it is desirable to suppress material degradation when the potential is high.
Various technologies have thus far been described that attempt to improve the performance of fuel cells by suppressing material degradation and the like when there is a fuel deficiency. For example, Published Japanese Translation of PCT application, JP-T-2003-508877 describes combining a water electrolysis catalyst with an electrode catalyst to minimize corrosion of the carrier of an anode catalyst when there is a shortage of hydrogen. The publication asserts that the resulting catalyst further increases resistance against battery reversal of a fuel cell.
Also, Japanese Patent Application Publication No. JP-A-2004-22503 describes an anode of a proton-exchange membrane fuel cell that includes at least one reaction layer in contact with a solid polymer electrolyte membrane and promotes a fuel cell reaction and at least one water splitting layer in contact with a diffusion layer and decomposes water in the anode using an electric current. The publication asserts that the described technology provides an anode of a proton-exchange membrane fuel cell that inhibits a decrease in the electrode characteristics even when there is a shortage of fuel.
Further, Japanese Patent Application Publication No. JP-A-2005-149742 describes a catalyst carrier electrode for a proton-exchange membrane fuel cell, which has a metal-supported catalyst, in which a catalyst metal is carried on a catalyst metal carrier that uses conductive metal oxide that is highly resistant to corrosion. This publication asserts that it possible to maintain power-generating performance of the fuel cell even of the cathode is exposed to a high potential state. In addition, Japanese Patent Application Publication No. JP-A-2005-135671 describes an electrode and the like that is formed of at least catalyst metal particles, a catalyst carrier, the main component of which is two or more types of carbon with different electron conductivities, and a proton conducting member. The electrode includes more of the catalyst carrier having the highest electron conductivity than it does of the other catalyst carrier(s). This publication asserts that corrosion of carbon is suppressed in the resulting electrode so that deterioration of electrode performance prevented.
In addition, Japanese Patent Application Publication No. JP-A-2005-141966 describes a catalyst carrier electrode with an electrode catalyst layer that includes a catalyst metal carrier conductive member in which a catalyst metal is carried on a conductive carrier, and an electrolyte polymer. The catalyst carrier electrode is characterized in that the electrode catalyst layer contains an electrolyte polymer and/or a conductive carrier containing water repellant material. The publication asserts that the described technology ensures that water and gas are able to pass through the catalyst layer by having the layer of water repellant material be sacrificed and broken up so that the water repellant material is released over time. As a result, corrosion of the catalyst layer from water is effectively prevented. Furthermore, Japanese Patent Application Publication No. JP-A-2005-294264 describes a membrane electrode assembly with a cathode catalyst layer that includes a composite of platinum black and a carried catalyst. The publication asserts that the described technology provides a fuel cell that degrades little and thus has a long life while maintaining battery performance.
The technologies described in JP-T-2003-508877 and JP-A-2004-22503 are able to suppress the corrosion of the anode constituent material (such as carbon) by promoting a water electrolysis reaction. However, the technologies in JP-T-2003-508877 and JP-A-2004-22503 only go so far as to implement measures for the anode as countermeasure technologies for when there is a deficiency of fuel. That is, JP-T-2003-508877 and JP-A-2004-22503 make no mention of technology for implementing measures for both the anode and the cathode as countermeasure technology for when there is a deficiency of fuel.
The following regarding countermeasure technology for when there is a deficiency of fuel was discovered by the inventors through intense study.
1) The anode is exposed to a high potential state when there is a shortage of fuel (hereinafter referred to as “hydrogen”) supplied to the anode side.
2) While hydrogen is supplied only to the hydrogen inlet area as hydrogen starts to be supplied to the anode side when the anode side returns from the hydrogen deficient state, a so-called partial battery is formed because the hydrogen has not yet spread to the hydrogen outlet area (hereinafter also referred to as “the anode outlet area”). When this happens the potential at the portion of the cathode opposite the anode outlet area across the electrolyte membrane increases so the cathode becomes exposed to a high potential state.
3) When a sufficient amount of hydrogen is supplied to the anode side, the anode reaction and the cathode reaction take place in the PEFC, which brings the cathode and the anode out of the high potential state.
The technologies described in JP-T-2003-508877 and JP-A-2004-22503 are countermeasure technologies for the anode side so they are able to suppress degradation of the anode constituent material in the case of 1) above. However, with respect to 2) above, neither JP-T-2003-508877 nor JP-A-2004-22503 make any mention of 2) above so the technologies described in those publications are unable to prevent degradation of the cathode material which is caused by 2) above. When the cathode material degrades, it is more difficult for the cathode reaction to take place. As a result, the performance of the fuel cell declines. That is, with the technologies described in JP-T-2003-508877 and JP-A-2004-22503 it is difficult to suppress a decline in performance caused by a fuel deficiency.
Also, neither JP-A-2005-149742 nor JP-A-2005-294264 make any mention of 2) above. Therefore, it is difficult to prevent degradation of the cathode material that is caused by 2) above.
Here, even if there is a shortage of hydrogen supplied to the anode while the fuel cell is operating, such that the anode is in a hydrogen deficient state, the operating environment of the fuel cell is normally controlled by a control apparatus or the like. Therefore, before long hydrogen is supplied to the anode thus bringing it out of the fuel deficient (hereinafter also referred to as “hydrogen deficient”) state. That is, the countermeasure for 1) above alone is insufficient as a countermeasure for the hydrogen deficient state. Only by implementing the countermeasures for both 1) and 2) above does it first become possible to effectively suppress a decline in performance in the fuel cell that is caused by a shortage of hydrogen.